The present invention relates to a method for the progressive detensioning of a prestressing cable.
The invention relates more particularly to a method allowing the controlled release of the energy accumulated in an external prestressing cable extending within a civil engineering structure along a path in the form of a broken line, the singular points of which consist, in particular, of anchorages or deflectors of the cable.
During checking operations carried out periodically on such civil engineering structures, structural faults in the component elements of the cables are sometimes detected, and it is therefore necessary to carry out their replacement.
Several replacement techniques are generally employed, but they differ according to the type of cable used.
Thus, a distinction is made between cables with non-adhesive injection and cables with adhesive injection.
For the first group of cables, a first technique involves using the extra cable length protruding in the region of the anchorage block in order to attach a tensioning jack. A second technique involves carrying out localized heating successively on each of the wedging pieces in the region of the anchoring head, in order to allow the cable to slip and relax. This second technique is often used when there is no extra cable length (sawn-off anchorage). One disadvantage of this technique is that there is a risk of an abrupt recoil of the anchoring pieces.
For the second group of cables, detensioning usually takes place by means of a heating technique in the running part of the cable (between the singular points) which has previously been stripped of its sheath and of the injection grout.
The main difficulties of this operation are:
the splashes or emanation of gaseous products during the stripping of the sheath and of the peripheral grout;
an obstruction of the slipping of the cable along its route;
possible jamming at the crossings of the singular points of the structure;
an instantaneous release of the energy stored in the cable in the event of an uncontrolled fracture of the cable, which may occur, in particular, if some strands of the cable are damaged in the region of the working zone, something which is not known beforehand (the remaining strands may break abruptly during the removal of the sheath and of the grout or during heating);
a whipping of the cable against the structure;
a considerable recoil of the anchoring pieces.
The uncontrolled fracture of a cable may have serious or even disastrous consequences. Prestressing cables store considerable energies, the abrupt release of which may seriously damage the structure and endanger the safety of the operating personnel.
An object of the present invention is to overcome these disadvantages by providing a safe method for the progressive detensioning of a prestressing cable with adhesive injection.
Accordingly, the invention provides a method for demounting a prestressing cable of a structure, the cable comprising a sheath, at least one tendon extending in the sheath and tensioned between two anchoring devices, and a hardened material filling the sheath around said at least one tendon.
According to the invention, the method comprises the following steps:
installing at least one energy dissipation device between a zone of the structure and an accessible portion of the cable, the energy dissipation device having a first end applied to a bearing zone and a second end, and being suitable for limiting the force to which it is subjected as a function of the speed of relative displacement between said first and second ends, said portion of the cable being adjacent to a first member for retaining the cable;
clamping said portion of the cable in a second retaining member engaging the sheath and connected to the second end of the energy dissipation device; and
severing the cable between the first and second retaining members.
By virtue of these arrangements, a prestressed cable can be detensioned without risking damaging the civil engineering structure and in complete safety for the user.
In preferred embodiments of the invention, it is possible, where appropriate, to resort to any of the following features:
anchoring the first retaining member to the structure;
applying the first end of the energy dissipation device to a bearing zone formed by a zone of the structure, or alternatively by said first retaining member;
using the energy absorbed by the energy dissipation device when the cable is detensioned responsive to the severing step for driving back the second retaining member in the direction of the position which it occupied before the cable was severed.
According to another aspect of the invention, using a cable detensioning method of the above-mentioned type, there is provided a device for demounting a prestressing cable of a structure, the cable comprising a sheath, at least one tendon extending in the sheath and tensioned between two anchoring devices, and a hardened material filling the sheath around said at least one tendon. According to the invention, the device comprises at least one energy dissipation device having a first end to be applied to a bearing zone and a second end, and being suitable for limiting the force to which it is subjected as a function of the speed of relative displacement between said first and second ends, and a second retaining member connected to the second end of the energy dissipation device, in order to engage the cable sheath in an accessible portion thereof located adjacent to a first member for retaining the cable.
In preferred embodiments of the demounting device, it is possible, where appropriate, to resort to any of the following features:
the energy dissipation device comprises at least one hydraulic cylinder/piston system extending between the second retaining member and the bearing zone, the demounting device further comprising a circuit for bringing towards an accumulator chamber a hydraulic fluid expelled in response to the relative displacement between the piston and the cylinder, said circuit comprising a flow-limiting member;
the accumulator chamber houses a first fluid brought to the accumulator chamber in response to the relative displacement between the piston and the cylinder and a second, gaseous fluid, and comprises a free surface separating said first and second fluids to ensure equilibrium between the pressure of said first and second fluids;
the energy dissipation device limits the hydraulic fluid being transferred between the cylinder and the accumulator chamber;
the second retaining member comprises a jaw having a clamping capacity of up to 40 tons.